Numerous studies have shown that humans classify colors into a restricted number of categories that appear to be universal across cultures. The results of most studies are, however, heavily dependent on the choice of color terms made available to the subjects. Moreover, most studies used monochromatic stimuli, yielding results difficult to compare to those of the electrophysiological literature. We thus designed a new technique to measure the boundaries of color categories, and used it to investigate the classification of colors in a physiologically relevant color space (MacLeod-Boynton). During training, our subjects (N=17) learned to associate a color name with each of four white shapes (square, disc, rectangle, or diamond) presented a dark background. Upon presentation of one of the four shapes, the subjects had to name it as quickly as possible with its associated color name. We measured the subjects' verbal reaction time for the correct naming of the four shapes. For the data collection proper, the task remained the same except that the shapes were presented not only in white (the control condition), but also in various colors (all of equal luminance). The stimuli subtended 1–3 deg of visual angle, had a luminance of 32 cd/m2, and were presented for 150 msec on a background with a luminance of 2 cd/m2. When the color of the stimulus was congruent with its associated color name, the reaction times were shorter than for the control. The reaction times grew proportionally with the difference between the stimulus physical color and its associated color name, similar to the well-known Stroop effect. The curves relating the subjects' reaction times to the color of the stimulus yield estimates of the boundary for each color category tested. These results are compared to those obtained with standard color categorization paradigms, as well as to the color tuning of single neurons in the visual pathways.